For micro-electromechanical systems (MEMS) and bio-MEMS (e.g., micro total analysis systems, or μTAS), a variety of force actuation methods have been used to either move structures or to induce stress in stationary structures, including electromagnetic, electrostatic, pneumatic, thermal, piezoelectric, magnetostrictive, among others. Each of these actuation technologies has advantages, disadvantages, and trade-offs in terms of performance metrics such as force capability, total displacement (stroke), actuation bandwidth, ease of integration to particular applications, and manufacturability.
Electrostatic actuation is the most commonly employed technology in commercial applications that require actuation forces produced within the lateral plane of the lithographically fabricated device, though some are employed to tilt objects about a pivot in some applications, notably mirrors in Texas Instruments' Digital Light Processor chip [TI 2006]. Conventional electrostatic actuators rely on the property that a virtual displacement of one electrode towards another generates a decrease in overall energy. The energy change is achieved through a decrease in the thickness of the dielectric (air). One problem with this type of actuator is that the actuation stroke depends on the thickness of the dielectric (the air gap) while actuation force is dependent on it inversely. Thus, large forces and large strokes cannot be simultaneously achieved.
In MEMS electrostatic actuators, a decoupling of actuator stroke from dielectric thickness has been achieved by using a comb arrangement as disclosed in inventions like Rodgers, et al., U.S. Pat. No. 6,133,670, hereby incorporated by reference. In comb actuators, an air gap serves as the dielectric and its thickness is the gap between comb teeth (typically 1 micron or greater). But this arrangement suffers from limitations of its own. As the force produced by electrostatic comb actuators is increased by decreasing comb teeth spacing or increasing the length of the teeth, the device becomes prone to side instability (i.e., instability perpendicular to comb teeth and desired stroke). In addition, practical limits of lithography also limit the teeth spacing and hence, force capability.
The actuators in Wapner & Hoffman, U.S. Pat No. 6,152,181, hereby incorporated by reference, rely on changes in the shape and position of liquid droplets in response to external stimuli to operate pumps, valves and other mechanical devices like sensors. These devices take advantage of changes in surface tension and capillary pressure to actuate. They rely on the principle of electrowetting which refers to the change in wetting behavior of a liquid when an electric potential is applied [Shapiro, 2003].
Other actuation methods in the prior art illustrate that the wetting effect can be enhanced by the use of a dielectric layer, see [Moon 2002, Shapiro 2003] as a reference. Electrowetting has been investigated for the movement of fluid droplets in μTAS [Pollack 2002] or integrated circuit cooling systems, for altering the shape of a liquid lens or liquid mirror, and in optical devices [Jackel 1983]. In these applications, electrowetting is employed for the translational motion of a fluid drop across a surface, or for the distortion of drop shape so as to affect the drop's optical properties. In droplet transport, an imbalance in surface tension forces induces droplet motion; motion is not caused by a change in capillary pressure. A number of patents on these applications exist. Electrowetting has also been used to move a solid mirror floating on top a liquid metal drop by distorting the drop's shape via increased wetting of the drop on a flat supporting substrate; the mirror in this case moves normal to the supporting substrate [Wan 2006]. With the application of voltage, the mirror moves to a new equilibrium position where the total force exerted by the drop upon the mirror is unchanged from the value achieved with no applied voltage. (In equilibrium, the drop force must equal the gravitational force acting upon the mirror which is unchanged.)
A main problem with the prior art uses of electrowetting for actuation is, but not limited thereto, that it is of limited application. Specifically, it is limited to applications that rely only on the change in shape or the change in position of the liquid. These systems depend on gravity (in the case of the floating mirror) or the physical structure of the surrounding system, in the case of sensors and valves, to enable actuation after the liquid responds to the applied potential.
The prior art fails to provide a means to produce and sustain significant force or pressure with continuous applied voltage. Furthermore, capillary pressures achieved in the prior art are, by necessity, greater than ambient pressure. The prior art fails to leverage the fact that at small scale (for example, less than 1 millimeter), the forces of capillary pressure and surface tension can be very large, making this actuation scheme particularly appealing for micro-electromechanical systems (MEMs) and bio-MEMS, as well as for larger mesoscale (for example, approximately 1 mm) devices. By combining together many such small capillary force actuators, powerful meso- and macroscale actuators can, in principle, also be developed.
There is a need to provide an actuator that is capable of optimizing the various performance metrics of actuators for improved performance in existing systems and for the facilitation of new actuation-based systems currently unavailable due to the limitations in the prior art. Notably, there is a need for actuators that provide increased force capabilities at reduced voltage levels. There is also a need for out-of-plane forces in relation to the substrate in lab-on-a-chip, and other MEMS (i.e. force creation in a direction other than parallel to the surface) without sacrificing stability, force capability, or actuator stroke, and for greater stability and wear-resistance in actuators that are currently employed in mechanical systems.